The present invention relates generally to level detection circuits, and more particularly to a level detection circuit which holds the maximum peak value of an input signal and accordingly detects the maximum value of the level of the input signal.
Recently, in cassette type tape recorders, a circuit is known which automatically adjusts and establishes the most suitable recording bias current, recording equalizer characteristic, recording and/or reproducing sensitivity, and the like, according to the kind of tape used by employing a micro-computer or the like to perform the measurements, as disclosed in U.S. Pat. Nos. 4,220,979, 4,258,397, and 4,279,005, for example. When effecting this type of an automatic measurement, there is a need to record a specific signal on a part of the tape, and reproducing this signal to detect the level. Therefore, it is necessary to employ a level detection circuit which is capable of accurately detecting the signal level.
The conventional level detection circuit which employs a micro-computer to detect the level was organized so that an input signal is applied to the integrating circuit to perform an integration operation by charging a capacitor in the integrating circuit during a specific time controlled by the micro-computer, the charged electric charge in the above capacitor is discharged after the above specific charging time is elapsed, and the discharging interval during the voltage of the capacitor reaches a specific voltage (for example, zero volts) is measured by the micro-computer, to detect the input signal level by using the above time.
However, when the contact between the magnetic tape and the magnetic head is instable due to instability of the magnetic tape travelling in the tape recorder, fluctuation is introduced in the reproduced signal level. Therefore, the integrated voltage varied due to the fluctuation in the signal level, and suffered a disadvantage in that accurate level detection could not be performed. Furthermore, upon partial drop-out of the reproduced signal, the integrated voltage varied, and also suffered a disadvantage in that accurate level detection could not be performed.
Hence, the inventor of the present invention perceived on the point that the level fluctuation of the reproduced signal in the tape recorder is generated symmetrically up-and-down as in the regular amplitude modulation, and that even upon the above fluctuation in the reproduced signal level, its maximum value is substantially equal to the maximum value of the reproduced signal upon normal contact between the magnetic tape and head, and level upon level fluctuation in other parts is lower than the normal reproduced signal level. Accordingly, in the conventional circuits, the whole reproduced signal level was integrated, thus not being able to perform accurate level detection. However, in the present invention, instead of measuring the fluctuating reproduced signal level, only a maximum level portion which occurs during a specific period of time is measured selectively.